In the audio industry today as well as the car industry for installing audio products, the quality of sound reproduced by loudspeakers is improving at a drastic speed with the prevalence of digital products. In these industries, the trend in loudspeakers is to advance high sound quality, weight reduction and harmonization with the environment.
With regard to the high sound quality, there is an important and pressing need for development of diaphragms, dust caps and sub-cones, which are the principal components of loudspeakers having significant degrees of influence in determining the sound quality, in order to satisfy the demand of users for the quality of sound. In development of these diaphragms, the research and development are being made with priority given to beaten type papers for the reason of easiness of controlling the sound quality more accurately.
FIG. 9 is a flowchart showing a method of manufacturing diaphragms and dust caps for loudspeakers using the conventional wet-lay papermaking.
As shown in FIG. 9, pulp used as the material of diaphragms and dust caps for loudspeakers is supplied to a beater filled with water, and it is finely beaten for a few days. The pulp then undergoes a chemical treatment process, in which it is admixed with a dyestuff, a binder, a fixing agent and the like substances, followed by a papermaking process comprising the steps of pouring the pulp into a vat, diluting, dispersing and wet-laying to form a beaten sheet of paper. Thereafter, a certain proportion of water in the sheet is evaporated in a dewatering process, or the first step of the drying process, and the sheet is then thermally compressed with a press in a compressing process to further promote evaporation of the water while improving the uniformity. Finally, the sheet is punched with a cutting die in a cutting process, or the last step, to remove an outer peripheral portion not necessary for a loudspeaker diaphragm and a center hole portion for insertion of a voice coil, to produce the loudspeaker diaphragm formed of the beaten sheet of paper. In the like manner, the sheet is punched with a cutting die to remove an outer peripheral portion not necessary for a loudspeaker dust cap, to thus produce the loudspeaker dust cap formed of the beaten sheet of paper.
When necessary, a sound quality conditioner may be additionally coated or impregnated by means of dipping or spraying between the compressing process and the die-cutting process, or after the die-cutting process. The above processes complete the conventional loudspeaker diaphragm and the loudspeaker dust cap.
Although the method of manufacturing the loudspeaker diaphragm and the dust cap discussed above includes the step of compressing the sheet of paper after the papermaking process, there also exist some un-pressed loudspeaker diaphragms and dust caps not subjected to the compressing process. There is not a considerable difference, however in the method of using the sound quality conditioner for making the un-pressed loudspeaker diaphragms and dust caps.
The raw material to be beaten in FIG. 9 is a cooked, unbleached or bleached pulp as used hitherto. If loudspeaker diaphragms are manufactured from fibers that are beaten only with a beater or the like equipment, they are weak in strength and small in modulus of elasticity, so that they bear a number of problems for use as the loudspeaker diaphragms with respects to the strength as well as their sound quality. Various studies have been made in the efforts of resolving the above problems. Patent document 1 below is one of the prior art documents known to be related to such studies.
Kraft pulp obtained from coniferous trees through the cooking process has hitherto been used as the material of beaten papers for diaphragms and dust caps, and this tends to accelerate the shortage of coniferous trees. It is for this reason that the use of environmentally friendly materials is necessary and indispensable in the future. Patent document 2 is known to be related to one such study among other prior art documents.
The diaphragms, dust caps and sub-cones composed of beaten papers, mainly of such paper materials as the kraft pulp obtained from coniferous trees, tend to have low rigidity in general due to inherent properties of the materials, as compared to diaphragms, dust caps and sub-cones composed of metallic materials and resin materials. It is difficult to improve rigidities of the conventional diaphragms, dust caps and sub-cones composed of papers due to the material nature. Loudspeakers made of these diaphragms, dust caps and sub-cones therefore have a shortcoming that they are not suitable for reproduction of sound with high clarity, which is one of the features essential to high sound quality, high power output and high reliability.    Patent document 1: Japanese Patent Unexamined Publication, No. 2003-230197    Patent document 2: Japanese Patent Unexamined Publication, No. 2000-324591